1. Field of the Invention
This invention pertains to a method for fabricating a hybrid IC (integrated circuit) substrate and particularly to a method for fabricating electric conductors integrated with high density on an insulator substrate.
2. Description of the Prior Art
According to a conventional method for forming a conductor layer by plating on an insulator substrate, a catalyzer (e.g., palladium) is absorbed on the whole surface of the substrate and then electroless plating is applied thereto or electroplating is also applied thereafter, so that a conductor layer is formed on the whole surface. After that, a resist layer in a predetermined pattern is formed on the conductor layer and a conductor pattern is formed by etching. (Afterwards, the resist layer is removed.) This conventional method makes it possible to obtain good adhesion of the conductor to the substrate if substrate surface treatment is appropriately applied before absorption of the catalyzer in the case of using a substrate of a composite material such as glass-epoxy. The substrate surface treatment is for example a treatment in which the surface of the substrate is roughened mechanically by using a brush or the like. However, if ceramic such as alumina is used for the substrate, it is difficult to improve the adhesion of the conductor to the substrate by roughening of the surface of the substrate.
FIG. 1 is a partial schematic sectional view of a ceramic circuit substrate disclosed in Japanese Patent Laying-Open No. 112681/1984. In FIG. 1, a tungsten conductor 102 belonging to the first group of conductors is formed on a ceramic substrate 101. The tungsten conductor 102 is covered with a nickel film 103 and a palladium film 104. A thick conductor film 105 of silver-palladium belonging to a second group of conductors is electrically connected with the palladium film 104 and with the tungsten conductor 102 through the nickel film 103. The nickel film 103 and the palladium film 104 are provided to improve adhesion between the tungsten conductor 102 and the palladium conductor 105.
The ceramic circuit substrate of FIG. 1 is fabricated in the below described process. First, the tungsten conductor 102 is provided on a green sheet of ceramic and it is baked in a reducing atmosphere, whereby the ceramic substrate 101 is formed with the tungsten conductor 102 being fixedly stuck thereto. The nickel film 103 is formed on the tungsten conductor 102 by electroless plating and the palladium film 104 is formed on the nickel film 103 by electroplating. After that, silver palladium paste containing a glass component is printed in a predetermined pattern and it is baked in an oxidizing atmosphere at 900.degree. C. so as to be firmly stuck to the ceramic substrate 101 and thus the thick film of the conductor 105 of the second group electrically connected with the tungsten conductor 102 is formed.
However, since the silver palladium paste containing the glass component is provided by screen printing, the patterning precision of the thick film conductor 105 is limited by a precision of screen printing. Further, there is involved a disadvantage that the silver palladium is liable to dissolve into fused solder.